1. Field of the Invention
The present invention relates to a method for making composite structures, and more particularly relates to a method for making smooth surface profile composite structures.
2. Description of Related Art
A glass fiber reinforced composite product must demonstrate appropriate mechanical properties such as tensile, flexural and impact strength and possess a smooth, defect free surface on at least one side to replace parts formed from sheet metal panels. The suitability of using reinforced thermoplastic resin composites for vehicle body parts is currently being investigated. The sheet metal currently used for wide, thin body parts, such as the hood or trunk of an automobile, is a likely candidate for replacement by a glass fiber reinforced composite material.
Generally, such a glass fiber reinforced composite body part would be manufactured by providing layers of thermoplastic resin fiber mat in the desired quantity and structural arrangement to form a laminated structure; by heating the laminate to a temperature in the range of about 200.degree. C. to about 375.degree. C., depending on the thermoplastic resin system, and by applying a pressure to the laminate in the range of about 5 lb/in.sup.2 to about 80 lb/in.sup.2, thereby forming a composite material blank and by shaping the composite blank by flow forming or compression molding processes to form the desired vehicle body part. While a composite material manufactured by such a process possesses the mechanical and flexural strength required of a vehicle body part, the high quality, smooth defect-free surface finish that is also required to meet automotive requirements for exterior body applications has been difficult to produce by the use of existing composite structures and processing techniques. A general discussion of existing processes for producing and for compression molding composites may be found in "Composites", Chou, T., et. al., Scientific American, Vol. 255, No. 4, October 1986, pp. 192-203, and Krone, J. R., and Walker, J. H., "Processing Thermoplastic Advanced Composites", Plastics Technology, Vol. 32, No. 11, November 1986, pp. 61-5.
In a typical flow forming process the composite blank is heated in a conventional oven by convection or infrared radiation to a temperature in the range of about 200.degree. C. to about 375.degree. C., depending on the thermoplastic resin. During the initial heating in the oven the fibers expand, resulting in a resin poor coating of the composite surface. In addition, this expansion of the fibers results in a lofting, or movement, of the fibers into the resin surface layers.
Following the oven heating, the composite is transferred to the mold where it is shaped by applying pressure in the range of about 1000 lb/in.sup.2 to about 5000 lb/in.sup.2 with mold surfaces whose temperatures range from about 55.degree. C. to 150.degree. C. During the transfer of the composite from the oven to the mold the composite surface cools and the surface resins "freeze" into position with a glass rich rough surface. This "freezing" of the resin at the surface prevents the resin from flowing readily during the molding process and, consequently, rough boundaries are produced between the newly formed surface areas and the original surface areas. In addition, the resulting composite surface is only partially filled with resins, even though some hot resin will move from the composite core to the surface during the molding process. This partially filled resin surface, particularly around and near the lofted fibers, is a major cause of surface roughness.
This problem of surface roughness is particularly troublesome for composites of crystalline thermoplastic resins because crystalline thermoplastic resins exhibit substantial shrinkage during cooling thereby projecting fibers at the surface of the composite.
It is an object of the present invention to provide a method for reducing the surface roughness of composites containing crystalline thermoplastic resin.